From d699ba41349e4ef397222a7223606fa03f4c4870 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Mon, 10 Feb 2014 15:14:07 +0200
Subject: [PATCH] Fix WakeupWaiters() to not wake up an exclusive locker
unnecessarily.
WakeupWaiters() is supposed to wake up all LW_WAIT_UNTIL_FREE waiters of
the slot, but the loop incorrectly also woke up the first LW_EXCLUSIVE
waiter, if there was no LW_WAIT_UNTIL_FREE waiters in the queue.
Noted by Andres Freund. This code is new in 9.4, so no backpatching.
---
src/backend/access/transam/xlog.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 7f63185b1cc..508970a751d 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -1842,15 +1842,14 @@ WakeupWaiters(XLogRecPtr EndPos)
slot->xlogInsertingAt = EndPos;
/*
- * See if there are any waiters that need to be woken up.
+ * See if there are any LW_WAIT_UNTIL_FREE waiters that need to be woken
+ * up. They are always in the front of the queue.
*/
head = slot->head;
- if (head != NULL)
+ if (head != NULL && head->lwWaitMode == LW_WAIT_UNTIL_FREE)
{
proc = head;
-
- /* LW_WAIT_UNTIL_FREE waiters are always in the front of the queue */
next = proc->lwWaitLink;
while (next && next->lwWaitMode == LW_WAIT_UNTIL_FREE)
{
@@ -1862,6 +1861,8 @@ WakeupWaiters(XLogRecPtr EndPos)
slot->head = next;
proc->lwWaitLink = NULL;
}
+ else
+ head = NULL;
/* We are done updating shared state of the lock itself. */
SpinLockRelease(&slot->mutex);
--
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